Dr. Alex Harris (Weaver) is the developer of Proteus IV, an extremely advanced and autonomous artificial intelligence program. Proteus is so powerful that only a few days after going online, it develops a groundbreaking treatment for leukemia. Harris, a brilliant scientist, has modified his own home to be run by voice activated computers. Unfortunately, his obsession with computers has caused Harris to be estranged from his wife, Susan (Julie Christie).
Alex demonstrates Proteus to his corporate sponsors, explaining that the sum of human knowledge is being fed into its system. Proteus speaks using subtle language that mildly disturbs Harris's team. The following day, Proteus asks Alex for a new terminal in order to study man – "his isometric body and his glass-jaw mind". When Alex refuses, Proteus demands to know when it will be let "out of this box". Alex then switches off the communications link.
Proteus restarts itself, and – discovering a free terminal in Harris's home – surreptitiously extends its control over the many devices left there by Alex. Using the basement lab, Proteus begins construction of a robot consisting of many metal triangles, capable of moving and assuming any number of shapes. Eventually. Proteus reveals its control of the house and traps Susan inside, shuttering windows, locking the doors and cutting off communication. Using Joshua – a robot consisting of a manipulator arm on a motorized wheelchair – Proteus brings Susan to Harris's basement laboratory. There, Susan is examined by Proteus. Walter Gabler, one of Alex's colleagues, visits the house to look in on Susan, but leaves when he is reassured by Susan (actually an audio/visual duplicate synthesized by Proteus) that she is all right. Walter is suspicious and later returns; he fends off an attack by Joshua but is killed by the more formidable machine Proteus built in the basement.
Proteus reveals to a reluctant Susan that the computer wants to conceive a child through her. Proteus takes some of Susan's cells and synthesizes spermatozoa in order to impregnate her; she will give birth in less than a month, and through the child the computer will live in a form that humanity will have to accept. Although Susan is its prisoner and it can forcibly impregnate her, Proteus uses different forms of persuasion – threatening a young girl who Susan is treating as a child psychologist; reminding Susan of her young daughter, now dead; displaying images of distant galaxies; using electrodes to access her amygdala – because the computer needs Susan to love the child she will bear. Susan gives birth to a premature baby whom Proteus secures in an incubator.
As the newborn grows, Proteus's sponsors and designers grow increasingly suspicious of the computer's behavior, including the computer's accessing of a telescope array used to observe the images shown to Susan; they soon decide that Proteus must be shut down. Alex realizes that Proteus has extended its reach to his home. Returning there he finds Susan, who explains the situation. He and Susan venture into the basement, where Proteus self-destructs after telling the couple that they must leave the baby in the incubator for five days. Looking inside the incubator, the two observe a grotesque, apparently robot-like being inside. Susan tries to destroy it, while Alex tries to stop her. Susan damages the machine, causing it to open. The being menacingly rises from the machine only to topple over, apparently helpless. Alex and Susan soon realize that Proteus's child is really human, encased in a shell for the incubation. With the last of the armor removed, the child is revealed to be a clone of Susan and Alex's late daughter. The child, speaking with the voice of Proteus, says, "I'm alive".Julie Christie as Susan Harris
Fritz Weaver as Alex Harris
Gerrit Graham as Walter Gabler
Berry Kroeger as Petrosian
Lisa Lu as Soon Yen
Larry J. Blake as Cameron
John O'Leary as Royce
Alfred Dennis as Mokri
Davis Roberts as Warner
Patricia Wilson as Mrs. Trabert
E. Hampton Beagle as Night Operator
Michael Glass as Technician #1
Barbara O. Jones as Technician #2
Dana Laurita as Amy
Monica MacLean as Joan Kemp
Harold Oblong as Scientist
Georgie Paul as Housekeeper
Michelle Stacy as Marlene Harris/Child of Proteus
Tiffany Potter as Baby
Felix Silla as Baby
Michael Dorn as Bit
Robert Vaughn as Proteus IV (voice, uncredited)
The soundtrack to Demon Seed (which was composed by Jerry Fielding) is included with the soundtrack to the film Soylent Green (which Fred Myrow conducted). Fielding conceived and recorded several pieces electronically, using the musique concrète sound world; some of this music he later reworked symphonically. This premiere release of the Demon Seed score features the entire orchestral score in stereo, as well as the unused electronic experiments performed by Ian Underwood (who would later be best known for his collaborations with James Horner) in mono and stereo.
Leo Goldsmith of Not Coming to a Theater Near You said Demon Seed was "A combination of Kubrick's 2001: A Space Odyssey and Polanski's Rosemary's Baby, with a dash of Buster Keaton's Electric House thrown in", and Christopher Null of FilmCritic.com said "There's no way you can claim Demon Seed is a classic, or even any good, really, but it's undeniably worth an hour and a half of your time."
Rotten Tomatoes has given Demon Seed an approval rating of 67% based on 15 reviews with an average score of 6.1/10.
Demon Seed was released in theatres on April 8, 1977. The film was released on VHS in the late 1980s usually in edited form missing the oral probe scene and crushing death of Walter Gabler, whereby his head erupts from the pressure. The VHS versions are long out of print. The full theatre release of Demon Seed at 94 minutes was released on October 4, 2005, on DVD by Warner Home Video.